KCharSelect's internal model: ensure rowCount() is 0 for valid indexes

Summary:
This isn't a tree model.
QAbstractTableModel's hasChildren() is a bit stupid about asking the
model, I'll fix that in Qt: https://codereview.qt-project.org/258965
Still, this is more correct.

Caught by QAbstractItemModelTester, but probably harmless with
the view used here.

Test Plan:
not much yet, I'm debugging the assert that happens later on
(bug 395181)

Reviewers: cfeck

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20506